Export GOROOT for minibp / primary builder
Export GOROOT for minibp / primary builder

Otherwise they'll fall back to the GOROOT embedded in the distribution,
which may not be correct.

This used to be propagated through build.ninja.in, but exporting it in
the environment should catch any other Go users too.

Change-Id: I3d6de8494b266e9b127c0a528d31df03ddc5c3a0

danw/blueprint,danw/blueprint,google/blueprint,google/blueprint